The present invention relates to electromagnetic surveying. In particular, the present invention relates to the generation of electromagnetic (EM) fields when surveying for hydrocarbon reservoirs or other formations identifiable by their electrical properties.
Determining the response of the sub-surface strata within the earth's crust to electromagnetic fields is a valuable tool in the field of geophysical research. The geological processes occurring in thermally, hydrothermally or magmatically active regions can be studied. In addition, electromagnetic sounding techniques can provide valuable insights into the nature, and particularly the likely hydrocarbon content, of subterranean reservoirs in the context of subterranean oil exploration and surveying.
Traditionally seismic techniques are used during oil-exploration expeditions to identify the existence, location and extent of reservoirs in subterranean rock strata. Whilst seismic surveying is able to identify such structures, the technique is often unable to distinguish between the different possible compositions of pore fluids within them, especially for pore fluids which have similar mechanical properties.
Whilst oil-filled and water-filled reservoirs are mechanically similar, they do possess significantly different electrical properties, which can be discriminated by active EM surveying. Active EM surveying is based on generating an EM field with a suitable EM source positioned near to the seafloor. Energy from the source then propagates by diffusion through the subterranean strata and is measured by remote receivers arranged at or near the seafloor. The term “active” is used to differentiate from the passive EM technique of magneto-telluric (MT) surveying which measures the response of subterranean strata to EM fields generated naturally in the earth's upper atmosphere.
The standard EM source used for active EM is an electric dipole antenna streamed from a towed submersible often referred to as an underwater towed vehicle (UTV) or a remote operated vehicle (ROV), the latter term being used in the following. The electric dipole is driven by a suitable electrical waveform supplied from the survey vehicle, i.e. from topside. High power is important for the EM source since the diffusive EM signal that propagates through the subterranean strata is strongly attenuated resulting in the signals received at the detectors being weak. For the antenna to generate a powerful signal, the ROV needs to receive a high power signal from topside through a transmission cable. The normal difficulties of transmitting a high power electric signal along a cable then apply, such as transmission loss, timing drift, impedance matching and so forth.
A known design [1] is to use an AC generator and a step-up transformer topside and convert the electrical waveform into a high voltage/low current signal for transmission from topside to ROV, thereby reducing transmission losses along the cable. Following transmission to the ROV, the high voltage/low current signal is converted back into a low voltage/high current signal which is then used to drive the streamed antenna. The streamed antenna comprises two electrodes spaced along the length of the antenna and which are separated from each other by seawater when submerged. However, although the seawater provides a path for current flow between the electrodes, it also provides a load with an inherently high inductance. In practice, this has proved problematic, since the high inductance of the load causes the generation of a back EMF when the current is switched. The effect of the back EMF is to produce voltage transients that can be damaging to components within the waveform driving signal source.
Another significant design criterion for the ideal active EM source is the waveform it is capable of producing. The ideal EM source would be capable of generating any arbitrary functional form. In practice, a square wave (or rectangular wave) is an important profile to be able to produce. This is of interest not only for the fundamental frequency, but also for the higher order harmonics that arise, as will be understood from the Fourier expansion of a square wave. In other words, a square wave source can be exploited as a multi-frequency source. It is therefore important to be able to produce a clean square wave with well defined amplitude and timing specifications.
The known cyclo-converter device [1] operates by performing full-wave rectification of an input AC waveform over a predetermined number of cycles to produce a frequency doubled positive polarity full-wave rectified waveform. After the positive polarity full-wave rectified waveform has been produced, the rectification polarity is reversed for a further predetermined number of cycles to produce a frequency doubled negative polarity full-wave rectified waveform. Together the positive and negative polarity full-wave rectified waveforms provide an approximation to a square-wave waveform.
Periodic switching of the rectification polarity can be performed at half-wavelength multiples of the input AC to provide a square-wave approximation waveform having a fundamental frequency corresponding to the input AC waveform, by detecting and counting the number of zero crossings that occur in the input AC waveform. Such a square-wave approximation waveform has two main frequency components, namely: a lower fundamental frequency component, which corresponds to the frequency of the polarity switching, and a higher frequency component, which is a multiple of the input AC signal frequency.
Through extensive and successful use, the limitations of the known cyclo-converter design have become apparent. Variations in phase and amplitude of the AC signal generated by the antenna are undesirable. These problems arise in large part due to drift and variable attenuation during the transmission of the AC signal from topside to ROV. The timing of the polarity switching at the ROV is dependant on the phase stability of the AC signal, and this can be degraded by inductive and capacitive effects in the transmission from topside. For example, degradation of the AC signal can produce erroneous zero-crossing points or cause non-detection of a real zero-crossing point, which may in turn result in a polarity switching event being triggered at an incorrect instant. Thus, we have concluded that effective operation of the cyclo-converter requires good stability control of the AC signal supplied to it and, as far as possible, mitigating any instability effects by additional design features in the ROV. Furthermore, the transition period of the square-wave approximation from positive to negative polarity, and vice versa, is dependant on the frequency of the supplied AC. This has led us to design specialist high frequency generation equipment in order to drive the cyclo-converter rapidly so as to reduce the transition period.